Opening a tank venting valve during the operation of an engine and analyzing a reaction from a fuel/air ratio control loop for diagnosis is known. The fuel vapor mixed with air from the tank ventilation (purge gas) causes a disturbance of the control loop so that the occurrence of the disturbance indicates an operable tank ventilation and accordingly an operable tank venting valve in particular. Such a method is derived, for example, from German Patent Application No. DE 100 43 071. However, if a disturbance of the control loop does not occur due to a change in mixture, it is not possible to obtain clear information concerning the operability of the tank venting valve. Purely in principle, it is possible that the mixture that is fed to the internal combustion engine via an intact tank venting valve corresponds exactly to the same mixture that is fed to the internal combustion engine in any case. In this case, more extensive diagnosis is required. To this end, for example, diagnostic methods may be provided for testing actuators in the regulation and/or control of operating parameters in connection with idle-speed regulation, as derived, for example, from German Patent No. DE 39 14 536.
However, more extensive diagnoses of this kind may in part only be performed at idle-speed; in addition, they are often very time-intensive. Furthermore, they are often subject to error. As a result, the frequency of diagnosis may be limited.
An object of the present invention is therefore to provide a method for testing the operability of a tank venting valve to the end that a diagnosis is also possible when not idling, thereby greatly increasing the frequency of diagnosis. Furthermore, the method should be very immune to error.